JP7212664B2 - Metal refining method and metal refining apparatus - Google Patents

Description

The present invention relates to a metal refining method and the like for removing specific elements from aluminum-based molten metal by vacuum distillation.

BACKGROUND ART Lightweight aluminum-based members are being used in various fields as environmental awareness is heightened. By reusing scrap rather than using newly smelted (and further refined) aluminum, it is possible to promote the use of aluminum-based members while achieving significant energy savings and environmental load reduction.

However, various elements other than Al may be mixed in raw material molten metal (also referred to as "Al-based molten metal") obtained by melting scrap. In order to prepare a melt of a desired composition, it is necessary to remove unnecessary or excessive elements from the raw material melt. One of the purification methods is a vacuum distillation method (reduced pressure distillation method, vacuum degassing method, vacuum treatment method). The vacuum distillation method is a method of preferentially evaporating and separating (desorbing) elements having a higher vapor pressure than Al (eg, Zn, Mg, Pb, H, etc.) from the Al-based molten metal. The following documents (Patent Documents 1 to 5 and Non-Patent Document 1) have a description related to this.

JP-A-7-41879 JP-A-9-316558 JP-A-11-256251 Japanese Patent Application Laid-Open No. 2001-294949 Japanese Patent Application Laid-Open No. 2002-339024 JP-A-7-318257 JP 2014-215021

Otaki, Satsuki Onna, Mori, Kudo, Tanaka: Furukawa Electric Times, 104 (1999), 25

Patent Documents 1 to 5 and Non-Patent Document 1 all propose to evaporate and remove impurities such as Zn in a vacuum atmosphere from a raw material molten metal (Al-based molten metal) whose entirety is uniformly heated. There is nothing more. Patent Documents 6 and 7 describe a vacuum arc melting apparatus, but do not describe a vacuum distillation method at all.

SUMMARY OF THE INVENTION The present invention has been made in view of such circumstances, and an object of the present invention is to provide a method for refining an aluminum-based molten metal by removing specific elements by a method different from the conventional method.

As a result of intensive research to solve this problem, the present inventor succeeded in removing specific elements by a new vacuum distillation method in which the vicinity of the surface of an aluminum-based molten metal is locally heated. Developing this result led to the completion of the present invention described below.

《Metal refining method》
(1) The present invention is a metal refining method for refining the aluminum-based molten metal by removing specific elements contained in the aluminum-based molten metal by a vacuum distillation method, wherein local heating is performed to heat the vicinity of the surface of the aluminum-based molten metal. A metal refining method comprising steps.

(2) According to the metal refining method of the present invention (also simply referred to as "refining method"), specific elements can be efficiently removed from aluminum-based molten metal (also referred to as "Al-based molten metal"). The reason for this is considered as follows.

In the refining method of the present invention, the vicinity of the molten metal surface, which is the interface where the specific element moves (evaporates) from the liquid phase (Al-based molten metal) to the gas phase (above the Al-based molten metal), is locally heated. Therefore, the specific element can be efficiently evaporated without heating the entire Al-based molten metal to a high temperature. Specifically, it is not necessary to keep the Al-based molten metal at a high temperature, so it is possible to shorten the heating time, reduce the energy required for heating, and suppress the wear and tear (longer life) of equipment (furnace, etc.). becomes.

Also, when the molten metal surface is locally heated, convection occurs due to uneven temperature distribution in the Al-based molten metal. As a result, the specific element is continuously replenished to a locally heated region (referred to as a “locally heated region”), and the specific element can stably evaporate. A concentration distribution of the specific element may also occur in the Al-based molten metal due to the evaporation of the specific element. Therefore, the diffusion of the specific element can also replenish the local heating area with the specific element.

Furthermore, when the molten metal surface is locally heated, the surface film (oxide film, etc.) formed on the surface of the Al-based molten metal receives shocking thermal stress due to sudden volume changes and differences in thermal expansion. can be divided (destroyed) by Therefore, the specific element can stably evaporate from the newly exposed melt surface without being hindered by the surface film (oxide film, etc.).

(3) The refining method of the present invention may be used not only for recycling Al-based scrap but also for preparing various Al-based molten metals. However, by using the refining method of the present invention, it is also possible to efficiently obtain a recycled Al alloy from which specific elements have been removed from inexpensive scrap containing specific elements (such as Zn and Mg). Therefore, the metal refining method of the present invention may be grasped as a "recycled Al alloy manufacturing method". The reclaimed Al alloy from which the specific element has been removed may be used as a solidified product (ingot or the like) or as a molten metal (including a semi-molten state). Further, the recycling of Al-based scraps is not limited to cascade recycling, and may be upgraded recycling to wrought material or the like.

《Metal refining equipment》
The present invention is also understood as a metal refiner. The present invention includes, for example, exhaust means for evacuating the surface of the aluminum-based molten metal and local heating means for heating the vicinity of the surface of the aluminum-based molten metal, and the specific element contained in the aluminum-based molten metal is vacuumed. A metal refiner used for removal by distillation may also be used.

"others"
(1) The “vacuum distillation method” referred to in this specification is a vacuum atmosphere with a pressure lower than the atmospheric pressure at least on the surface side of the Al-based molten metal, and a specific element having a higher vapor pressure than Al is added to the Al-based molten metal. Evaporating from the surface side to reduce the specific element concentration in the Al-based molten metal. In this specification, the treatment (exhaust, condensation, recovery, etc.) of the specific element evaporated by the vacuum distillation method does not matter.

Also, the term “local” (locally heated region) as used herein may include at least a part of the surface (upper layer) of the Al-based molten metal. The local area may be the entire molten metal surface (the entire upper layer of the Al-based molten metal) or a portion of the molten metal surface (a portion of the upper layer of the Al-based molten metal). The term "near the surface of the molten metal" as used herein may include the surface of the molten metal, which is the interface at which the specific element evaporates. Suffice it to say, "near the surface of the hot water" is at least a partial area from the surface of the hot water to the predetermined depth. The predetermined depth may be, for example, about 1/3 of the depth of the molten metal.

(2) The aluminum-based molten metal referred to in this specification includes a solid-liquid coexisting state (semi-molten state). The aluminum-based molten metal may have any specific composition as long as Al is the main component (the Al content is more than 50 atomic %, 70 atomic % or more, or 85 atomic % or more with respect to the whole molten metal). Although the specific element concentration in the raw material molten metal (Al-based molten metal before refining) is not critical, it is usually about 10% by mass or less, further 5% by mass or less with respect to the entire molten metal. The concentration and composition referred to in this specification are indicated by mass ratio (% by mass or simply "%") with respect to the entire object (molten metal, etc.) unless otherwise specified.

(3) Unless otherwise specified, "x to y" as used herein includes the lower limit value x and the upper limit value y. A new range such as “a to b” can be established as a new lower or upper limit of any numerical value included in the various numerical values or numerical ranges described herein.

BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic diagram which shows the outline|summary (an example) of a metal refining apparatus. It is a schematic diagram which shows the structural example of a torch. It is a schematic diagram which shows the another example.

In addition to the components of the present invention described above, one or more components arbitrarily selected from this specification may be added. What is described herein may apply to equipment as well as methods of metal refining. A component relating to the method can also be a component relating to the object (apparatus, (reclaimed) Al alloy (melt), etc.).

《Local heating》
Local heating for heating the vicinity of the surface of Al-based molten metal (simply referred to as "molten metal") is preferably performed using a heat source with high energy density. As a result, it is possible to perform rapid heating to raise the temperature of the whole or part of the molten metal surface in a short period of time, and to efficiently evaporate and remove specific elements.

(1) Heat source The type, output, etc. of the heat source (apparatus) can be adjusted according to the form of the hot water bath (heating furnace, crucible, channel, etc.) containing the molten metal, the scale of refining (amount of molten metal), and the like. Examples of heat sources include high-energy beam irradiation (laser irradiation, electron beam irradiation, etc.), discharge (arc discharge, etc.), and the like. By using any heat source, it is possible to simplify the heating device, reduce the size, save energy, or reduce the load on the hot tub.

High-energy beam irradiation enables highly accurate identification of the heating position (local). Electric discharge can rapidly heat the vicinity of the molten metal surface by supplying high-temperature plasma. Discharge may be performed by arranging a pair of electrodes on the molten metal surface, or one electrode may be arranged on the molten metal surface and the other electrode (counter electrode) may be Al-based molten metal (molten surface). In the latter case, an electric discharge is generated between the Al-based molten metal (one electrode) and an electrode (the other electrode) placed above the molten metal surface, and the vaporization occurs near the molten metal surface (interface between the liquid phase and the gas phase). A specific area within a predetermined range is efficiently and rapidly heated. In particular, arc discharge, which is used in welding or the like, enables efficient local heating while reducing equipment burden.

(2) Energy density The amount of energy per unit area (energy density) imparted to the surface of the aluminum-based molten metal is 10 ² W/cm ² or more, 10 ³ W/cm ² or more, or 10 ⁴ W/cm ² or more. It should be above. In order to avoid an excessively large apparatus and bumping near the surface of the hot water, the energy density should be 10 ⁵ W/cm ² or less.

(3) Arc discharge Heating by arc discharge (arc heating) is due to a high-temperature arc column generated between electrodes. Although the temperature of the arc column varies depending on its location, it is at least higher than the flame (<3000°C), and reaches, for example, 4000°C or higher, or even 5000°C or higher (Tanaka: Welding Society, 77 (2008). ,50). The heating of the molten metal surface by the arc column may be direct arc heating or indirect arc heating. Arc heating is considered to be mainly due to radiation (radiation) and kinetic energy transfer of particles (electrons, plasma ions, etc.). The arc heating referred to in this specification includes arc plasma heating in which the arc discharge is constrained by a nozzle, an air current, or the like to increase directivity and heating temperature. Also, the power source for the arc discharge may be direct current or alternating current.

《Vacuum Distillation》
A specific element having a higher vapor pressure (saturated vapor partial pressure, which will be described later) than Al evaporates from the surface of the locally heated Al-based molten metal into a vacuum atmosphere above it (also simply referred to as "upper space").

(1) Vapor pressure Saturated vapor pressure depends on temperature and increases as temperature rises. The saturated vapor pressure (P ₀ ) of a pure metal simple substance (liquid phase) is expressed by the following equation (1) as a function of temperature (T) (source: Osafune: Bulletin of Tsuyama Technical College 13 (1975) 63).
logP ₀ =aT ⁻¹ +logT+cT+D (1)
where log: common logarithm, T: absolute temperature (K), a, b, c and D: constants.

In addition, the saturated vapor pressure ( _Pt ) of the entire molten metal composed of a plurality of constituent elements at temperature T is expressed by the following formula ( 2) (Source: Osafune: Bulletin of Tsuyama College of Technology 13 (1975) 63).
Pt = ΣP _i = Σ(α _i P _i0 ) (2)
Here, α _i : activity of the constituent element in the molten metal, P _i0 : saturated vapor pressure of the constituent element alone (liquid phase). The activity (α _i ) depends on the concentration of constituent elements in the molten metal and the temperature of the molten metal.

When the saturated vapor partial pressure (P _i ) of the constituent elements contained in the molten metal is higher than the partial pressure of the constituent elements in the sky, the constituent elements can evaporate (release, dissipate) from the surface of the molten metal to the sky. In the case of an Al-based molten metal, theoretically, each constituent element including Al can evaporate onto the molten metal according to the respective saturated vapor partial pressures and partial pressures on the molten metal surface.

However, the saturated vapor pressure of Al is negligibly lower than the saturated vapor pressures of other elements contained in the Al-based molten metal. This is the same as the saturated vapor partial pressure considering the activity. For example, the saturated vapor pressure at 700° C. is Al: 2×10 ⁻⁵ Pa, Zn: 8.4×10 ³ Pa, Mg: 7.5×10 ² Pa, Pb: 6.7×10 ⁻¹ Pa. be. Therefore, even if the activity (concentration) is considered, the vapor pressure (saturated vapor partial pressure) of other elements is about 10 ³ to 10 ⁷ times higher than that of Al. That is, there is a large vapor pressure difference between Al and the specific element in the Al-based molten metal. Therefore, in the local heating region, Al does not substantially evaporate, and the specific element having a high vapor pressure mainly evaporates. In addition, in this specification, the above-mentioned saturated vapor partial pressure is also simply referred to as "vapor pressure".

(2) Specific element The specific element to be vacuum-distilled (fractionated) may be an element having a large difference in vapor pressure (saturated vapor partial pressure) with respect to Al. For example, the specific element is one or more of Zn, Mg and Pb. A representative example of the specific element is Zn, which has a particularly high vapor pressure.

(3) Degree of vacuum The greater the degree of vacuum (lower atmospheric pressure) on the surface of the Al-based molten metal, the smaller the partial pressure of the specific element on the surface of the molten metal, and the greater the amount of evaporation of the specific element. The atmospheric pressure may be, for example, 0.1 to 10 ⁴ Pa, 1 to 10 ³ Pa, further 10 to 10 ² Pa. Too little vacuum slows the evaporation of certain elements. Realization of an excessive degree of vacuum causes a large equipment burden. The degree of vacuum (atmospheric pressure) is the average degree of vacuum (atmospheric pressure) above the hot water surface. The degree of vacuum (atmospheric pressure) is measured, for example, by a probe provided in a pipe extending above the surface of the molten metal.

(4) Airflow Vacuum distillation is preferably carried out while applying an airflow over the surface of the aluminum-based molten metal (especially the locally heated region). This suppresses the stagnation of the vapor of the specific element on the surface of the hot water (that is, the equilibrium state of the vaporization of the specific element), thereby ensuring continuous or stable vaporization of the specific element.

The airflow may be, for example, the flow (gas airflow) of gas (rare gases such as Ar and He, inert gases such as _N2 ) ejected from a nozzle or the like, or a plasma torch (simply referred to as "torch"). It may be a gas stream or a plasma stream ejected from the side of the generated electrode (torch electrode). The airflow is preferably upstream on the opposite side of the evacuation port with respect to the localized heating area.

The airflow is preferably along the melt surface, which is the evaporation starting point (boundary) of the specific element. Application of airflow may be continuous or intermittent, and may be synchronized with localized heating. In any case, there should be airflow at least in the localized heating area.

"purification"
Refining to remove specific elements from the Al-based molten metal may be batch processing (batch processing) or continuous processing (continuous processing). According to batch processing, the degree of vacuum in the processing chamber (closed space on the side of the hot water surface that is evacuated), the temperature near the hot water surface, the processing time, etc., can be set freely, so the concentration of specific elements can be sufficiently reduced. can do According to continuous processing, a large amount of Al-based molten metal can be efficiently processed, and other impurity removal processing and subsequent casting processes can be smoothly coordinated.

The purification (vacuum distillation) referred to in this specification is sufficient as long as the specific element concentration in the Al-based molten metal is reduced. Although the specific element evaporated from the Al-based molten metal may be discarded, it is preferable to capture, collect and reuse it.

《Metal refining equipment》
A metal refiner (also simply referred to as "apparatus") can take various forms depending on the type of heat source for local heating. A good example of this is when arc discharge is used as the heat source. An arc column (arc plasma) generated by arc discharge has a high energy density and can efficiently and rapidly heat the vicinity of the surface of the Al-based molten metal.

The local heating means using an arc discharge as a heat source is preferably provided with a torch electrode having a tip facing the surface of the Al-based molten metal. Electricity (voltage application) between the torch electrode and the Al-based molten metal can cause arc discharge between the tip of the torch electrode and the surface of the Al-based molten metal.

Electricity is supplied to the Al-based molten metal, for example, through a tundish made of a conductor such as a metal, or through a counter electrode at least partially immersed in the Al-based molten metal. For example, the counter electrode may be arranged above the Al-based molten metal together with the torch electrode. As a result, the structure and functions necessary for local heating are concentrated above the Al-based molten metal, making it possible to reduce the size of the apparatus, improve maintainability, and improve the workability of supplying and replacing the Al-based molten metal (tank).

The torch electrode is preferably surrounded by an insulator on its outer peripheral surface. As a result, free discharge is suppressed, and arc discharge generated between the tip of the torch electrode and the surface (local portion) of the Al-based molten metal can be stabilized. The free discharge is a discharge that can occur between the torch electrode (including the outer peripheral surface), the surface of the counter electrode, the wall surface of the bath, the surface of the Al-based molten metal, and the like.

The range in which the insulator surrounds the outer peripheral surface of the torch electrode may be a range in which free discharge can be suppressed. Normally, it is preferable that the insulator extends to the vicinity of the tip of the torch electrode.

The insulator may be cylindrical or tubular into which the torch electrode is inserted (inserted), or may be in the form of a film that covers the outer surface of the torch electrode. The insulator should be detachable from the torch electrode. This makes it possible to replace only one of the worn or damaged insulator and the torch electrode. The insulator may also serve as a holder for the torch electrode. Furthermore, the insulator preferably constitutes at least a part of the flow path of the gas supplied to the surface of the Al-based molten metal. These enable compactness and simplification on the torch side.

It is preferable to provide a holder that can attach and detach the insulator and that can airtightly hold the insulator. It is possible to replace only the insulator, which is easily consumed, and to maintain airtightness in the processing chamber. Also, if the insulator and the holder can be separated (divided), the insulator and the holder can be made of different materials depending on the respective arrangement environments. For example, the insulator disposed near the arc column or the Al-based molten metal to be locally heated should be made of a material (ceramics, etc.) excellent in heat resistance, low thermal expansion, thermal shock resistance, and the like. Since the holder is arranged apart from the arc column and the Al-based molten metal, it does not necessarily have to be made of a material that is as excellent in heat resistance as the insulator. The holder is preferably made of a material that can be machined (free-cutting ceramics/machinable ceramics, etc.) in order to airtightly hold the insulator.

The insulator and the holder are preferably made of at least ceramics with excellent insulating properties (for example, volume resistivity of more than 10 ⁵ Ω·cm). Such ceramics include, for example, Al ₂ TiO ₅ (aluminum titanate), SiO ₂ ·Al ₂ O ₃ , SiO ₂ , Al ₂ O ₃ and the like as main components (more than 50% by mass, or more than 75% by mass of the total). ). In particular, the insulator is preferably made of ceramics (for example, Al ₂ TiO ₅ ) that has a high dielectric breakdown voltage as well as the above-described heat resistance.

The arc discharge may be hot cathode arc or cold cathode arc. In any case, the torch electrode is preferably a cathode (negative electrode, cathode). At this time, it is preferable that the wall surface of the tundish or the processing chamber and the Al-based molten metal (counter electrode) are substantially at an electric potential. The electrode (torch electrode, counter electrode) exposed to high temperatures is preferably made of a high boiling point material such as carbon (C) or tungsten (W). Generally, graphite (electrode) is preferably used. Although the shape of the electrode is not limited, it is usually (cylindrical) columnar or (round) bar-shaped.

Vacuum distillation was performed by locally heating the vicinity of the surface of the Al-based molten metal containing Zn. The Zn concentration in the Al alloy obtained from the Al-based molten metal after the treatment was measured. The present invention will be described in more detail based on such specific examples.

[First embodiment]
"Device"
An outline of an impurity removal device S (simply referred to as “apparatus D”), which is an example of a metal refining device, is schematically shown in FIG.

Apparatus D comprises a holding tank 1 for heating and holding an Al-based molten metal m (simply referred to as "molten metal m"), and a processing chamber v (upper space) sealed in the holding tank 1 by closing the upper part of the holding tank 1. , an exhaust unit 3 for evacuating the inside of the processing chamber v, and a local heating unit 4 for heating the vicinity of the surface s of the molten metal m.

The holding tank 1 (heating furnace) includes a housing 11, a crucible 12 (hot water tank) containing the molten metal m, and a heater 13 capable of melting raw material metal (aluminum-based scrap, etc.) and adjusting the temperature of the molten metal m in the crucible 12. and The crucible 12 was made of alumina, and the heater 13 was of electric resistance type.

The exhaust unit 3 includes an oil rotary vacuum pump 31 (exhausting means), a regulating valve 32 for adjusting the degree of vacuum in the processing chamber v, and an exhaust filter 33 for trapping vapor, fine particles, etc. sucked from the processing chamber v. , an exhaust pipe 34 communicating with the processing chamber v.

The local heating unit 4 (local heating means) includes a power supply 40, a torch 41, and a counter electrode 42 for generating an arc discharge a. The torch 41 includes an electrode 411 (torch electrode) and a gas pipe 412 surrounding the electrode 411 . An inert gas (Ar) is supplied to the gas pipe 412 from a gas source (such as a cylinder) on the upstream side. The gas pipe 412 is preferably made of an insulating material such as ceramics.

A TIG (Tungsten Inert Gas) welding power source is used as the power source 40 . Both the electrode 411 and the counter electrode 42 were round rod electrodes, and the tip of the counter electrode 42 was immersed in the upper portion of the molten metal m. When electricity is supplied between the electrode 411 and the counter electrode 42 by the power source 40, an arc discharge a is generated between the surface s of the hot water and the tip of the electrode 411 (in the vicinity of the tip surface). At least part of the gas supplied from the gas pipe 412 is continuously plasmatized by the arc discharge a, and an arc column and a plasma flow are stably generated. A part of the inert gas discharged from the downstream side of the gas pipe 412 becomes a gas stream g along the outer circumference of the arc discharge a and the molten metal surface s. The gas stream g along the surface s diffuses the vapor generated on the surface s into the processing chamber v. A part of it is exhausted from the exhaust part 3, and the rest becomes liquid or solid as the temperature drops and deposits on the exhaust pipe 34 and the filter 33.

"purification"
Al-based molten metal (raw material molten metal) containing Zn (specific element) to be removed was purified using the apparatus D described above. Also, the Zn concentration contained in the Al alloy made of the molten metal after refining was measured. Specifically, it is as follows. The purification (vacuum distillation) conditions and Zn concentration for each sample are also shown in Table 1.

(1) Al-Based Molten Metal Al-1.2% Zn molten metal was prepared in the crucible 12 as an Al-based molten metal (raw material molten metal) before refining. The Zn concentration is the mass ratio of Zn to the entire molten metal or alloy. Commercially available pure Al and pure Zn were used as metal raw materials for the molten metal. The amount of Al-based molten metal used for each sample was 500 g. The temperature of the molten metal was measured at a depth of about 25 mm from the molten metal surface s, and the temperatures shown in Table 1 were obtained. The surface area of the melt surface s was approximately the same as the opening area of the crucible 12 and was 50 cm ² .

(2) Vacuum Distillation The vacuum pump 31 was operated to evacuate the sealed processing chamber v to reduce the pressure. The time when the inside of the processing chamber v reached the degree of vacuum (absolute pressure) shown in Table 1 was defined as the start of processing, and the elapsed time from that time was defined as the processing time.

For Samples 1 and 2, the power source 40 was energized, and local heating was performed by the arc discharge a from the start of the treatment. The arc discharge a was a DC arc with the electrode 411 as a negative electrode (cathode) and the counter electrode 42 as a positive electrode (anode). A graphite rod with a diameter of 3.2 mm was used as the electrode 411, and a graphite rod with a diameter of 4 mm was used as the counter electrode . The tip of the counter electrode 42 was immersed to a depth of about 50 mm from the surface s of the hot water. The discharge current accompanying the arc discharge a and the Ar gas flow rate forming the gas stream g were as shown in Table 1. The discharge time is the processing time shown in Table 1. The heating area near the molten metal surface s by the arc discharge a was about 3.1 cm ² , and the energy density imparted to the molten metal surface s by the arc discharge a was about 500 W/cm ² .

For the samples C11, C12 and C2, the conventional vacuum distillation method was performed without energizing the power supply 40 or supplying the gas to the surface s of the hot water.

(3) Measurement After stirring the entire amount of the Al-based molten metal of each sample after refining treatment, part of it was poured into a stainless steel analysis mold and solidified by natural cooling in the air. The chemical composition (Zn concentration) of the Al alloy thus obtained was measured by fluorescent X-ray spectroscopy, and Table 1 also shows the results.

(4) Evaluation As is clear from samples 1 and 2 shown in Table 1, when the molten metal surface s is locally heated, the Zn concentration can be efficiently increased in a short time without increasing the temperature of the molten metal or the degree of vacuum. It turned out that it can be reduced.

On the other hand, as is clear from the samples C11, C12, and C2 shown in Table 1, in the purification by the conventional vacuum distillation method, both the temperature of the molten metal and the degree of vacuum must be increased and the Zn concentration must be maintained for a long time. could not be reduced.

From the above, it can be seen that by locally heating the vicinity of the surface of the Al-based molten metal as in the present invention, the specific element can be efficiently removed from the Al-based molten metal in a short time without increasing the temperature of the molten metal or the degree of vacuum. rice field.

[Second embodiment]
Figs. 2 and 3 schematically show cross sections of essential parts of devices D1 and D2 in which the structure of the torch 41 arranged on the lid 2 is changed. The same reference numerals are given to the members already described, and the description thereof is omitted as appropriate. The up-down direction (vertical direction) and left-right direction are indicated by arrows in the figure.

"Device configuration"
(1) The torch 41 of the device D1 includes an insulating cylinder 413 (insulator) that covers the outer peripheral surfaces of the electrode 411 and the gas pipe 412 in the processing chamber v. The insulating cylinder 413 has a cylindrical shape having an annular stepped portion 4131 on the upper side, the diameter of which increases toward the outer peripheral side.

The insulating tube 413 is fixed to the lid 2 by bringing the upper surface 4131a of the stepped portion 4131 into close contact with the lower surface of the lid 2 and holding the lower surface 4131b of the stepped portion 4131 with a fastener 431 provided on the lower surface of the lid 2. will be served. Since the fastener 431 is screwed to the lid 2, the insulating tube 413 can be attached to and detached from the lid 2. As shown in FIG.

The upper end of the insulating cylinder 413 is covered with a disk-shaped holder 432 (holding body) having a stepped hole 4321 penetrating therethrough near the center. The holder 432 is fixed to the upper surface of the lid 2 with a plurality of screws 433 with the gas pipe 412 inserted into the stepped hole 4321 . O-rings 434 and 435 seal between the lower surface of the holder 432 and the upper surface of the lid 2 and between the inner peripheral surface of the stepped hole 4321 and the outer peripheral surface of the gas pipe 412, respectively. The holder 432 is machined to form annular grooves for accommodating the O-rings 434 and 435 . In this way, the insulating cylinder 413 can be attached to and detached from the holder 432, and airtightness can be maintained between them (and thus the processing chamber v).

(2) The torch 41 of the device D2 does not include the insulating cylinder 413 described above. A holder 442 fixed to the upper surface of the lid 2 has a straight hole 4421 penetrating through the center. The gas pipe 412 is held by being fitted through the straight hole 4421 .

"evaluation"
(1) Arc discharge
The vacuum distillation described above was performed using the apparatus D1 and the apparatus D2. However, the temperature of the molten metal was kept at 740° C., and energization (arc discharge) was started when the atmospheric pressure (degree of vacuum) in the processing chamber v became 650 Pa or less. Arc discharge was performed with a DC current of 100 A, a discharge time of about 200 seconds, and an Ar gas flow rate of 5 L/min.

Here, electrode 411: graphite electrode (φ3.2 mm), counter electrode 42: graphite electrode (φ6 mm). The electrode 411 was on the negative electrode side, and the counter electrode 42 was on the positive electrode side.

Further, the insulating cylinder 413 is made of ceramics (FTA-PS manufactured by Maruwai Yano Ceramics Co., Ltd.) mainly composed of Al ₂ TiO ₅ . The holders 432 and 442 are made of ceramics (photoveil) mainly composed of SiO ₂ ·Al ₂ O ₃ .

The stability of the arc discharge produced by each device was observed. In the case of the device D1, since the outer peripheral surface of the electrode 411 was surrounded by the insulating cylinder 413 up to the vicinity of the tip, a stable arc was generated without free discharge.

On the other hand, in the case of the device D2, since the insulating cylinder 413 was not provided on the outer peripheral side of the electrode 411, free discharge occurred between the outer surface of the electrode 411 and the wall surface of the holding tank 1 made of metal. Fluctuation was also confirmed in the arc formed at the tip of the electrode 411 .

From the above, it was found that by providing an insulator surrounding the outer surface of the torch electrode, suppression of free discharge and stability of arc discharge can be achieved.

(2) Durability Three insulating cylinders 413 made of different ceramics were prepared. The insulating cylinder 413 mainly made of Al ₂ TiO ₅ described above is Sample 1, the insulating cylinder 413 mainly composed of SiO ₂ ·Al ₂ O ₃ described above is Sample 2, and made of Al ₂ O ₃ (99.99% by mass). Assume that the insulating cylinder 413 is a sample 3 . All ceramics had a volume resistivity of more than 10 ⁵ Ω·cm.

The vacuum distillation described above was performed using the device D1 incorporating the insulating cylinder 413 for each sample. Discharge time: The durability (damage life) of each sample was evaluated with one discharge time of about 200 seconds. It should be noted that there was no electric leakage, free discharge, atmospheric pressure leakage, etc., when any of the samples (insulating cylinder 413) was used.

In the case of sample 1, the insulating cylinder 413 was not damaged even after performing vacuum distillation more than 20 times. In the case of sample 2, cracks were found at the tip of the insulating cylinder 413 after vacuum distillation was performed five times. In the case of sample 3, when the vacuum distillation was performed twice, a crack was confirmed at the tip of the insulating cylinder 413 .

The reason why the insulating cylinder 413 of Sample 1 was excellent in durability is considered to be that the ceramics made of Al ₂ TiO ₅ had a microcrack structure, and the thermal expansion was moderated and the thermal expansion was low.

From the above, if ceramics with excellent heat resistance, thermal shock resistance, low expansion, etc. are used for the insulator disposed in the processing chamber, vacuum distillation can be stably performed for a long time, and the vacuum distillation cost will be reduced. It was found that reduction can be achieved.

D impurity removal device 1 holding tank 12 crucible (hot water tank)
2 Lid 3 Exhaust Part 31 Vacuum Pump (Evacuation Means)
4 local heating unit 411 electrode (torch electrode)
413 insulating cylinder (insulator)
432 holder (holding body)
m Al-based molten metal s Surface a Arc discharge g Gas stream

Claims (15)

A metal refining method for refining the aluminum-based molten metal by removing specific elements contained in the aluminum-based molten metal by a vacuum distillation method,
A local heating step of heating the vicinity of the surface of the aluminum-based molten metal ,
The local heating step is a metal refining method in which gas is supplied onto the surface of the aluminum-based molten metal. 2. The metal refining method according to claim 1, wherein said local heating step is performed by applying energy of 10< ² >W/cm< ² > or more to the surface of said aluminum-based molten metal. 3. The metal refining method according to claim 1, wherein said local heating step is performed by arc discharge. 4. A metal refining method according to claim 3 , wherein said gas is supplied to the outer circumference of said arc discharge . 5. The metal refining method according to any one of claims 1 to 4, wherein the vacuum distillation method is carried out at a pressure of 0.1 to 10 ⁴ Pa above the surface of the aluminum-based molten metal. The metal refining method according to any one of claims 1 to 5, wherein the specific element is one or more of Zn, Mg and Pb. Exhaust means for evacuating the surface of the aluminum-based molten metal;
local heating means for heating the vicinity of the surface of the aluminum-based molten metal ;
and means for supplying gas onto the surface of the aluminum-based molten metal ,
A metal refiner used for removing specific elements contained in the aluminum-based molten metal by a vacuum distillation method. 8. The metal refining apparatus according to claim 7, wherein said exhaust means exhausts the inside of the processing chamber above the surface of said aluminum-based molten metal. The local heating means is
a torch electrode having a tip facing the surface of the aluminum-based molten metal;
and an insulator surrounding the outer peripheral surface of the torch electrode,
9. The metal refining apparatus according to claim 7 or 8, wherein an arc discharge can be generated between the tip of the torch electrode and the surface of the aluminum-based molten metal by energizing the torch electrode. 10. A metal refining apparatus according to claim 9, further comprising a holder capable of attaching and detaching said insulator and capable of airtightly holding said insulator. 11. The metal refiner according to claim 10, wherein said insulator and said holder are made of different materials. 12. The metal refiner according to claim 9, wherein said insulator is made of aluminum titanate. The metal refiner according to any one of claims 9 to 12, wherein the insulator constitutes at least part of a flow path for gas supplied to the surface of the aluminum-based molten metal. Furthermore, comprising a counter electrode partially immersed in the aluminum-based molten metal,
The metal refiner according to any one of claims 9 to 13, wherein the torch electrode and the counter electrode are arranged above the aluminum-based molten metal. The metal refiner according to any one of claims 9 to 14, wherein said torch electrode is made of graphite.

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